High quality multilevel halftoning for color images with reduced memory requirements

ABSTRACT

A method is disclosed for producing a colour image by printing on a sheet a plurality of monochrome images on top of each other. The monochrome image (37) is composed of microdots (36), having an address (x,y). Each microdot (36) is represented by a pixel (32). The pixel (32) carries information about the address (x,y) and an image signal  I  x,y that corresponds with the density to be printed on the microdot (36). This image signal is given as 8 bit signal data for example. The microdots (36) are partitioned by a screen (40) in identical screen cells (33), composed of M microdots (36). Each microdot is associated with a pixeltonecurve (34). The image signal  I  x,y is transformed to a bitmap signal B by the pixeltonecurve (34). The image signal  I  x,y is transformed to a bitmap signal B by the pixeltonecurve (34) corresponding to the microdot (36). The bitmap signal B is represented by 2 or 4 bit resulting in important memory savings for the bitmap signals. The bitmap signal B is further transformed to a printer signal. The printer signal is transformed by printing to a density on the microdot (36) with address (x,y).

DESCRIPTION

1. Field of the Invention

The present invention relates to a method for printing digital colourimages on hard copy and is intended in particular for use in desktop orgraphical applications like electronic printing, copying and colourproofing.

2. Background of the Invention

The last few years, electrophotography has made two importantevolutions. First of all the ability to fix on paper coloured toners ontop of each other, opened the way to make printed hard copies of colourimages. Secondly the increased number of density levels for one printeddot, started the move from binary printing to multilevel (4, 8, 16levels) or almost continuous tone (64, 128, 256 levels) printing. One ofthe most performing high-resolution colour scanner/printer/copiersystems on the market is the AGFA XC305 and AGFA XC315 system. Thissystem uses eight bit data for every printed dot for each of thesubtractive colour components (Cyan, Magenta, Yellow and blacK : CMYK).Every printed dot on printed matter can thus get theoretically 256different density levels for each colour component C, M, Y and K.Although some printers based on electrophotographic printing technologyclaim to be able to represent 256 different density levels per colour onone printed dot, this is an overstatement. Printers with anaddressability of 400 dpi (dots per inch) can effectively achieve 64 andexceptionally up to 128 different density levels per printed dot. Forthat reason said copier uses a screening technique, even when operatingfully in eight bit mode. It is known that printed matter with a screenruling of 200 lpi (lines per inch) and 128 grey levels can render highquality images.

Together with a spatial resolution of 400 dpi (dots per inch) for theindividual printed dots, this system is able to deliver high qualitycolour copies. These copies can be used for short-run colour, i.e. smallamounts of coloured copies ; and for Direct Digital Colour Proofing(DDCP), i.e. checking the colour output of a reproduction on anothersystem.

The printer/copier can be used in direct colour copy mode. In that case,the original colour document is scanned four times, for each printingphase C, M, Y and K respectively. For every phase, the three additivecolour components (Red, Green and Blue or RGB) of every pixel on theoriginal copy are simultaneously quantified into 8 bit data and combinedon the fly in the image manipulation unit within the copier to a signalrepresenting the subtractive component C, M, Y or K to be printed on thecopy. This process poses no big intermediate storage requirements,because the scanning speed is chosen to be conform with the processingspeed of the processor means in the image manipulation unit to combinethe RGB signals to the C, M, Y or K signals and the printing speed forthe electrophotographic process. Said processor means does not have tobuffer or store large portions of the image to be copied. Memory meansfor buffering two scanned lines will suffice in most cases.

Another feature of this type of printer/copier is that a colour image inelectronic representation format can be downloaded to the colourprinting system. This offers the possibility to connect it to a RasterImage Processor (RIP). A RIP is an electronic device that converts aPage Description Language (PDL) in a colour bitmap. For binaryblack-and-white printing systems, a simple bitmap storing one bit ofinformation per printed dot is enough. In colour output systems, onebitmap per colour component is necessary. In multilevel systems, thebitmap needs more than one bit of information per printed dot. A bitmapcontains the signals that can be stored in memory means like DRAM(Dynamic Random Access Memory) making a printed-dot-wise representationfor the image to be printed on the hard copy.

Said downloaded image can be formally described in terms of a PDL.AgfaScript (AgfaScript is a trade mark of Agfa-Gevaert A.G. inLeverkusen, Germany) and PostScript (PostScript is a trade mark of AdobeSystems Inc.) are examples of a PDL. A PDL offers commands for drawingcoloured graphics, printing coloured characters in various fonts withwhatever orientation and size and rendering coloured images representedby a rectangular two dimensional array of pixels. Because the system isable to output colour hard copies, the RIP can process colours and iscalled therefore a colour-RIP. The RIP converts PDL input files intoCMYK bitmaps.

As the PDL commands--to draw graphics, print a character or render animage--are fed sequentially to the colour-RIP, a bitmap for each colourto be printed, i.e. CMYK, is updated.

Printing four colours on an A3 sized sheet (297 mm×420 mm) at aresolution of 400 dpi in both X and Y direction, each coloured pixelspecified by eight bits per pixel, requires 4*29.5 MByte=118 MByte (1Mbyte=2²⁰ byte=2²³ bits) of memory. Because most DRAM memory componentscome in 1, 4 or 16 Mbit, the number 118 Mbyte is rounded up towards thenext power of two, i.e. 2⁷ or 128 Mbyte. The excess of ten MByte can beused for storing the RIP software.

Most manufacturers who offered 32 MByte RIP software or RIP systems,evolve to the larger memory capacities of 64 and 128 MByte, in order totake full advantage of the contone printing capability of modern A3 pageprinters. 128 Mbyte DRAM memory in a RIP causes more than fifty percentof its cost. Therefore several ways to reduce the amount of requiredmemory are looked for.

One method known in the art (see The Seybold Report on DesktopPublishing Vol. 7, Nr. 2, Oct. 1, 1992 page 9) is reducing theinformation per printed dot to one bit per colour, using the printer asa binary device. This reduces the amount of required memory with afactor 8, to 14.5 MByte for printing a CMYK coloured A3 page at 400 dpiresolution. On the printed output, this gives no quality degradation forgraphics and characters in the fully saturated colours C, M, Y, K, R, G,B and White. However, for graphics and characters in other colours andfor rendering continuous tone images, a screening method applied to thesignals is necessary to render enough colour variations. This makes ahalftone image from the contone data. The screening process effectivelydecreases the spatial resolution of the image, loosing the details fromthe original image in the hard copy and resulting in poor quality outputfor scanned images.

Another method known in the art to decrease the amount of memory by afactor of two without loosing any quality, is reducing the size of theprinted image to slightly bigger than an A4 format (sized 210 mm×297mm).

Yet another way to achieve the same memory savings of 64 MByte is toreduce the resolution in one dimension from 400 dpi to 200 dpi, keepingeight bits per pixel for every colour. This has the advantage that thememory requirements are reduced by a factor of two, but puts severerestrictions on the achievable quality for text and graphics. Thequality loss is not visible in the horizontal or vertical segments ofcharacters or graphics, but said segments look bumpy in a directionclose to the direction of higher resolution, resulting in a seriousquality degradation for text and graphics. Also images loose quality andalthough this is not so apparent, it can be noticed by the bear eye ofan attentive observer.

OBJECTS OF THE INVENTION

It is an object of the present invention to provide a two bit multilevelscreening method for the generation of a colour bitmap, rendering animage with an important quality improvement over 1 bit binary halftoneprinting. The method can be used for rendering colour graphics andcharacters and for rendering scanned continuous tone images with memoryrequirements that are only twice those for the moderate memory consumingbinary halftoning method and one quarter of the requirements for fulleight bit processing.

Another object of the present invention is to develop a multilevelhalftoning method requiring 4 bit data per printed dot per colour,without any quality loss in text and graphics and a nearly invisibleloss in image quality as compared to the full eight bit method,producing an output that is extremely close to full contone quality,reducing the huge memory requirements for full eight bit processing witha factor two.

The choice of the appropriate output levels for a video interface lookup table means guarantees the best achievable quality for both two bitand four bit multilevel screening.

Yet another object of the present invention is to offer an enhancedperformance or ping-pong mode for multipage output applications withoutthe need to double the amount of DRAM memory, enabling the RIP toprepare the second page of a document whilst the first page is stillbeing printed.

SUMMARY OF THE INVENTION

In accordance with the present invention, we describe here a method ofproducing a colour image by printing on a sheet a plurality ofmonochrome images on top of each other, using the appropriate colour foreach said monochrome image wherein:

each said monochrome image is composed of microdots each having anaddress (x,y)

each said microdot is represented by one pixel, each pixel carryinginformation about the address (x,y) and an image signal I_(x),y ;

all microdots are partitioned by a screen in identical screen cells,composed of M (M is an integer) microdots R_(i) ;

each microdot R_(i) is associated with a pixeltonecurve L_(i) ;

for each said pixel, the location of the microdot R_(i) is dictated bythe address (x,y), the image signal I_(x),y is transformed by thecorresponding pixeltonecurve L_(i), to a bitmap signal B;

said bitmap signal B is transformed to a P-bit printer signal (P is aninteger);

said printer signal is transformed by printing to a density on themicrodot with said address (x,y)

said bitmap signal B is represented by a 2 or 4 bit value, depending onthe required quality, memory savings and performance.

The images aimed in this invention can be synthetic images composed bye.g. computer programs, contone or halftone images, acquired by scanningoriginals or any other means, true colour, false colour--i.e. black andwhite images that get a different colour assigned for every grey value,in order to increase the number of perceptible levels--or black andwhite images, or even images that can be represented by just two colourcomponents.

Although we concentrate on the images, because they pose the highestquality requirements on the system, it is clear that on the printed pagealso separately generated graphics, line art and characters can bepresent, e.g. defined within a PDL.

A sheet can be plain paper or a transparent sheet, or photographic orthermographic paper.

The printing system can include preferably an electrophotographicprinter, but also inkjet printers have the capability to print more thantwo levels and can be used to implement the method of this invention.

A microdot is the smallest point that can be addressed on the sheet bythe printer. The dot printed on the microdot can have any form, but wewill idealise its shape to square or rectangular.

The address (x,y) can e.g. be given by counting the number of microdotsfrom the left edge of the page for x, and from the bottom edge for y.

Colour images accepted by a RIP in some PDL are traditionallyrepresented in three rectangular planes, one plane for each additivecolour Red, Green and Blue. This colour representation can be in theCIE-XYZ space, as well as CIE-Lab or whatever other representation. Wefurther take for granted that the RIP performed the necessary colourprocessing to generate signals representing the colour image in thecolour space matching with the printer, mostly CMYK. A PDL can alsoaccept directly images in CMYK representation. In that case, the colourimage is represented by four rectangular planes. For false colourimages, one rectangular plane will suffice.

We also take for granted that the RIP performed all processing necessaryto match the scale and orientation of the image information with theaddressability of the printer, e.g. 400 dpi. This scaling and rotationcan be done by techniques known in the art such as "nearest neighbourresampling" or "pixel replication", "linear and bilinear interpolation"and "cubic B-spline" and other convolution techniques.

The image signals have preferably values from 0 to 255. But all othernumber of input levels can be handled by the same method. The finaldensity resolution will not however be better than the number of levelsin the input image. In fact, the images that can be processed range from1 bit/pixel pure binary halftone images to 8 bit per pixel contoneimages. Some systems offer 10 or even 12 bits per pixel per colour torepresent high quality contone images. These images can be processedusing the same concepts behind the current invention.

Defining screen cells on a screen is a well known technique in graphicalprocessing of colour images, and is e.g. described in the U.S. Pat. No.5,155,599.

The phrase "identical" screen cell means that all screen cells have anidentical shape, orientation and size. Only the placement of the cell onthe sheet is different (translation over X and Y). Each screen cell hasthe same amount of M microdots, numbered in the same order for allscreen cells. That means that every microdot with the same relativeposition in a screen cell has associated the same pixeltonecurve.

The pixeltonecurve effectively transforms each of the CMYK signals ofthe contone input image individually to a bitmap signal that can bestored in the respective CMYK bitmap, until it is completely preparedfor printing, the latter being a real time process.

DETAILED DESCRIPTION OF THE INVENTION

The invention is described hereinafter by way of example with referenceto the accompanying figures wherein:

FIG. 1 is a block diagram of a system for application of the methodaccording to the present invention;

FIG. 2 is a schematic view of the Raster Image Processor (RIP);

FIG. 3 is a schematic representation of the rastering process applied inthe current invention;

FIG. 4 is a schematic view of a screen cell with sixteen microdots andthe corresponding pixeltonecurves;

FIG. 5 shows an example of linearly mapped pixeltonecurves, ascendingsimultaneously for each microdot in the screen cell;

FIG. 6 is an example of improved pixeltonecurves for use in themultilevel halftoning method of the current invention;

Referring to FIG. 1, a copier 25 for colour originals is shown. Theprinting unit 26 of said colour copier system 25 can be used as astandalone colour printer, whereas the scanning unit 27 can be used as acolour input device or colour scanner.

The copier 25 further contains an image manipulation unit 30 that isoperative when the system is used as a standalone copier.

To apply the method of the current invention, a raster image processor28 is coupled directly to the image manipulation unit 30, such that thecolour image signals from a scanned original can be transmitted from thescanner 27 to the RIP 28. These signals can be further sent to aninteractive graphical workstation 29, where the scanned colour image canbe observed and manipulated. Parts of this image together with otherimage data and graphical or text commands can be sent by the interactivegraphical workstation 29, using a PDL data stream, to the RIP 28 nowacting as a raster image processor, rather than an interface between thescanner 27 and the workstation 29. After conversion of the PDL datastream to four bitmaps, the bitmap signals are transmitted to the imagemanipulation unit 30 that sends them to the printing unit 26.

In FIG. 2 we look into more detail to the components of the RIP 28. Itmainly consists of a processor board 21 equipped with three piggy backdaughter boards 22, 23, 24. We distinguish the DRAM module 22, the VideoInterface module 23 and the Ethernet/SCSI network interface module 24.

The input data signals for the RIP are transmitted over the EtherTalk orTCP/IP network towards the Ethernet network module 24, making the RIP toact as a usual network printer. EtherTalk is mostly used in Macintosh(TM) (System 7.x) and IBM (TM) PC and compatibles (PC Windows 3.1)environment and TCP/IP networking mostly for IBM PC and Unix (TM)workstations. The input data stream consists of PDL commands and data.

The processor board 21 contains two MC 68040/25 MHz microprocessors (notshown) for processing the incoming signals. The processing means on thisboard performs all calculations, conversions such as colour management,scaling and rotation and screening to convert the PDL data stream inseveral bitmaps. It further controls the three daughter boards.Referring to FIG. 3 and 4 we will further down describe how theprocessor means transforms the contone pixel data materialised by theimage signals to bitmap signals B.

The Video Interface Daughter board 23 acts as a memory bus module andhas two functions. In scanning mode, it interfaces with the scanningunit 27 of the copier 25 via the image manipulation unit 30. In onesingle scanning pass, the video interface 23 captures the signals forthe three scanned colour components, red, green and blue, at a totalrate of 40 Mbyte/sec. A field programmable gate array (FPGA on the videointerface 23, not shown) rearranges and packs the scanned data per fourpixels in three longwords of four bytes each in the format (RRRR GGGGBBBB) and sends the signals in burst access mode as three consecutivelongwords in DRAM memory means 22.

In the output or RIP mode, once all bitmaps are formed and stored in theDRAM memory means 22, the signals are sent via the video interface 23 tothe printing engine 26. For the scanner/printer used in conjunction ofthe current invention, said printing engine accepts P=8 bits per pixelonly. Other systems may accept six bit printing data signals, yet otherlower performant systems accept four bit per pixel etc. So for bitmapsin the format of 1, 2 and 4 bits per pixel, a conversion to 8 bits (6 or4 etc. for other systems) per pixel is necessary. More generally, allprinting systems will accept P bits per pixel, and allow L differentlevels, with L≦2^(P). E.g. P could be 8 and L could be 200 for a systemthat offers 200 density levels. The transformation from K bit bitmapsignals to P bit printer data is done via a video interface look uptable (VI-LUT, #41 in FIG. 3) means on the Video interface daughterboard 23.

The video interface daughter board 23 comprises 4 downloadableLUT's--one for each printing colour Cyan, Magenta, Yellow and blacK(CMYK)--each having 256 entries of 8 (P=8) bit data for conversion ofK=1, 2, 4 or 8 bit bitmap signals on the fly to P=8 bit data C, M Y or Kfor the printer device 26.

The information in the bitmap for the current invention can thus beavailable in K=1, 2, 4 or 8 bits per pixel. The pixel depth depends, aswe will discuss further down, on the amount of memory installed, thesystem performance and the quality required by the user.

The VI-LUT's convert the bitmap signals (1, 2, 4 or 8 bit) into 8 bitprinter data for the engine (4×8 bit CMYK). The easiest conversion islinear, with a minimum output value of 0 and a maximum output value 255.

E.g. in an 8 bit to 8 bit conversion, 0, 1, 2, . . . , 255 will bemapped to 0, 1, 2, . . . , 255.

In a 2 bit to 8 bit conversion, 0, 1, 2, 3 will be mapped to 0, 85, 170,255 respectively.

To get consistent maximum colour output on different copiers, themaximum output value can be lowered to a certain standard value e.g. 205which is 20% less than 255. The LUT's for the 2 bit system then may map0, 1, 2, 3 to 0, 68, 136, 205. In this way all engines can produce thesame maximum density for each colour. To obtain this goal, some engineshave to be configured to a higher maximum output LUT value to reach thesame (standard) density level.

For improved human perception and stability of the electrophotographicprocess, LUT's are mostly not set up with equidistant values asdescribed above. In a two bit system, 0, 1, 2, 3 will be mapped to E₀,E_(MIN), E_(STAB), E_(MAX). The background and explanation for this canbe found in the Belgian patent application 09300713 filed on Jul. 12th1993 titled "Rastermethode voor een schrijfsysteem met beperktedensiteitsresolutie".

The DRAM module 22 must store the various bitmaps and is--for a smallportion--also used as CPU working memory to load and run the system andRIP software. In the preferred embodiments of the current invention, wehave RAM modules of 32, 64 and 128 MByte. In order to justify thischoice, we first define the concepts of multilevel halftone screeningand enhanced performance mode.

The method of the current invention fills the gap between 1 and 8 bitper pixel processing with 2 and 4 bits per pixel. This means that 4 or16 density levels are to be selected for all printed dots. For bothoptions, the pixel data will be screened in order to obtain enoughdensity levels by human visual integration. The method used thereto iscalled multilevel halftone screening. Even with 4 bits per pixel, thesystem delivers crisp and clear images, that can be compared with the 8bit per pixel images, and graduated fills are smooth.

Another option offered by the method of the current invention is theperformance enhancement operating in ping-pong mode. The performanceboost is highly dependent on the complexity of the page currentlyprocessed from PDL to bitmap representation and the multiplicity ofcopies of the previous page. The possibility of holding two bitmaps witha lower pixel depth instead of one with a higher pixel depth is furthercalled the enhanced performance mode EPM. For short run applications,making more copies (typically 15 to 20) of the same bitmap, the timeT_(p) needed to print all the copies (typically 13 seconds per copy) canbe used to acquire and process the next page. Depending on thecomplexity of the next page, said processing or interpreting the PDLtakes a time T_(i) (sometimes up to ten minutes) longer or shorter thanT_(p). If T_(p) <T_(i), then T_(i) for all different pages must be addedup to get the time for the total job. If T_(p) >T_(i), then T_(p) mustbe added. If no parallel processing is possible, the sum of T_(p) +T_(i)must be added, because printing and interpreting is done purelysequential. As we will discuss in the preferred embodiments for memorysizes, EPM can be used, but at the cost of some print quality.

The system of the current invention thus offers the choice to selectbetween quality and speed. The EPM is only useful when printingsubsequent pages. The obtained quality of the output reaches anacceptable level even when printing with 4 bits per pixel. This mayencourage the user to give priority to the faster 4 bit per pixelprinting mode as opposed to the better quality 8 bit per pixel mode.

When the emphasis is put onto speed, the RIP will always try to get 2pages in memory, to enable EPM or ping-pong operation. While one page isread out, the next page can already be converted to bitmap data. Thismode is preferable when speed prevails over quality.

The first embodiment with 128 MByte DRAM gives the option to handle anA3 sized image at full 8 bit with a resolution of 400 dpi in bothdirections.

When 4 bit processing is acceptable, one can boost the systemperformance by processing the same A3 page in 4 bit mode. In that case,the four bitmaps for CMYK need only 59 MByte together, leaving another64 MByte for simultaneous EPM processing.

With this amount of memory, A4 documents can even be processed in fulleight bit mode and enhanced performance mode.

All documents smaller than A4 can be handled using EPM.

The second embodiment with 64 MByte DRAM allows printing of a full A3page, where normally 128 MByte is required. This big memory saving isachieved by the four bit multilevel halftoning technique of the currentinvention. Thus, A3 colour pages can be handled, not only in binaryhalftone screening, but also with 4 bit multilevel halftoning, whichgives a tremendous quality improvement over binary halftoning.

This embodiment also allows 1, 2, 4 and 8 bit printing for paper sizesup to A4. Moreover, the system with this configuration can handle an A4page in EPM using 4 bit. Printing an A4 is normally done at 8 bit perpixel. However, when 4 bits per pixel is acceptable, the bitmap memoryis capable of holding another A4 at 4 bit per pixel. This means that,when printing a first page, the bitmap can hold also the contents of thefollowing page. In this way, the RIP's speed is increased with animportant factor, applying EPM.

All documents smaller than A5 can be processed in full eight bit modeand with EPM.

The third embodiment proposes a system with 32 MByte DRAM.

An A3 page can be printed by binary screening.

An A4 page can be printed by the 2 bit multilevel halftoning method. Fortext and graphics, the 2 bit option gives a freedom of 64 differentcolours. Images screened with a two bit multilevel screening method showan important quality improvement over traditionally binary screenedimages. With a relative low amount of memory (32 MByte), the RIP offersmore versatility and an improved image quality. If EPM is necessary, 1bit processing or binary screening for A4 must be accepted.

All documents smaller than A5 can be handled in full eight bit mode.

The next table summarises the above mentioned possibilities of theembodiments in a system with a 32, 64 and 128 MByte configuration. Thelast column "scanning surface" will be discussed later in this text.

    ______________________________________                                                                           Scanning                                           A5     A4         A3       surface                                    ______________________________________                                        32 MB     8 bit    2 bit      1 bit  Max A5                                                      EPM 1 bit                                                  64 MB     8 bit    8 bit      4 bit  Max A4                                             EPM 8 bit                                                                              EPM 4 bit                                                  128 MB    8 bit    8 bit      8 bit  Max A3                                             EPM 8 bit                                                                              EPM 8 bit  EPM 4 bit                                       ______________________________________                                    

Now we will discuss how the processor means performs the multilevelscreening process.

Referring to FIG. 3, a monochrome image 31 is represented by individualpixels 32, carrying information about the address (x,y) and an imagesignal I_(x),y. At the right side of FIG. 3, the sheet 37 is shown,partitioned in microdots 36 by a screen 40, grouping the microdots inidentical 4×4 (M=16) screen cells 33.

The processing of the image signals is as follows. A clock generator 38generates a clock signal with a frequency that imposes the processingspeed of the incoming PDL data stream. This clock signal is sent to theaddress generator module 39. At the rhythm of the incoming clock signal,the address generator module 39 generates simultaneously a signal x anda signal y. On each new clock pulse, another combination (x,y) isgenerated, that corresponds with the address of a microdot 36 on thesheet 37 and thus the address of the corresponding pixel 32. The signalsx and y are sent to the processor, handling the input data stream. Byreceiving the address (x,y), the processor will fetch from the input PDLdata stream the image signal I_(x),y corresponding to pixel 32,describing the density value for microdot 36. The image signal I_(x),yis sent to the appropriate pixeltonecurve L_(i) (34). As will bedescribed in FIG. 4, the correct pixeltonecurve L_(i) is selected byexamining the address (x,y), materialised by the x and y signals. (x,y)determines the microdot R_(i) within the screen cell, and each R_(i) hasa specific pixeltonecurve L_(i) associated. The image signal I_(x),yindexes in the pixeltonecurve L_(i). The pixeltonecurve L_(i) therebygenerates the bitmap signal B. That signal is represented by K bits, thevalue of K depending on the multilevel method used. If K=1, then themethod is simply binary halftoning. If K=2, we have the four levelhalftoning technique of the current invention. If K=4, as shown in FIG.3, then we apply the multilevel halftoning method with sixteen levels. Kcan also be chosen to be eight, in which case the method is fully eightbit.

The K bit bitmap signals B can now be stored in DRAM memory means or anyother means, until the bitmaps are completely composed. Then the bitmapsignals B are sent to the appropriate video interface look up table 41(VI-LUT) within the video interface daughter board 23. For eachmonochrome image CMYK there is a different VI-LUT, possibly withdifferent output signals. The VI-LUT transforms the K bit bitmap signalsB into P bit printer signals. For the current example with K=4, only thefirst sixteen entries of the VI-LUT 41 need to be filled with signals,because B is only indexing these values. The output of the look up tablemeans 41 is now an eight bit signal (P=8 in this example) that istransmitted directly to the printing device 35. The printer devicetransforms the P bit printer signals into a density on a microdot 36 onthe sheet 37 at the location with address (x,y).

FIG. 4 illustrates how a screen cell 61 with sixteen microdots 60 ishandled. Each microdot R_(i) gets a different pixeltonecurve L_(i) 62,depending on the relative position of the microdot 60 within the screencell 61. The pixeltonecurve is represented here as a table 62 permicrodot 60 with 256 table entries, because we suppose that the inputcontone image data I_(x),y are given as N=8 bit signals. The signal 63stored in that table entry is the bitmap signal B. For binary screeningtechniques, B can only take two values: 0 and 1. For the multilevelhalftoning method of the current invention, B can take four or sixteendifferent values, depending on whether the system is operating as a twobit or four bit system (K=2 or 4). Let us concentrate here on thelatter. That means that B has any value from 0 to 15. The value B issent to and stored as a signal in the DRAM memory means, and isretrieved as it is required for printing at a later stage.

In FIG. 5 we show diagrams representing the pixeltonecurve signals,combined with the VI-LUT signals for a four bit or sixteen levelhalftoning method. In fact sixteen levels from 0 to 255 are chosen withequal increments. The image signal I=0 (or I_(x),y) is transformed tothe bitmap signal B=0 by all pixeltonecurves. I=1 is transformed to B=1for all pixels belonging to microdot R₁, and to 0 for all others. ForI=2, both L₁ and L₂ generate B=1, other L_(i) still generate 0. ForI=16, all pixeltonecurves generate B=1. The output signals for I=17 arekept the same as for I=16. For I=18, L₁ generates 2, all others stillgenerate 1, etc. . . The K=4 bit bitmap signals B are linearlytransformed to P=8 bit printer signals by the VI-LUT mapping 0, 1, . .15 to 0, 17, . . 255. At the right hand side of FIG. 5, we show thecumulative sum of the sixteen curves belonging to the individualmicrodots. The top curve 16 gives an idea of the mapping between imagesignals I and the visual density, integrated over the 4×4 screen cell.

FIG. 6 shows sixteen 4-bit pixeltonecurves combined with the VI-LUT,corresponding each to a microdot in a 4×4 screen cell, especiallyoptimised for an electrophotographic process. Each curve shows the samesixteen discrete levels, that are chosen to be not equidistant, but cannevertheless be represented by 4 bit signals. Because for each basiccolour the system can render at least 64 density levels per printed dot,sixteen density levels can be chosen adequately from the available 64for optimal human perception and stability of the electrophotographicprocess. The need for this level selection and a preferred way aredescribed in the above mentioned Belgian patent application 09300713.

As described above, the images that can be printed using the concepts ofthis invention, can be scanned images. The scanner/printer/copier XC305that can be used in conjunction with the system of the currentinvention, always scans the data with 8 bit signals for Red, Green andBlue at a fixed resolution of 400 dpi.

The three colour components RGB are offered simultaneously at a rate of13.3 MB/sec for each component. The system acquiring the scanned data isfree to acquire one, two or three colour components simultaneously.Acquiring them in one pass gives the advantage that the three resultingmonochrome images are very accurately registered with each other. Thisis a problem when acquiring the scanned colour components separately inthree passes. To take advantage of the single pass scanning, the systemmust be able to process the data at a rate of 40 Mbyte/sec.

Because most interactive graphical workstations acquire images via SCSI,that speed cannot be supported. Also the memory requirements, shown inthe table above under scanning, are huge.

The RIP system of the current invention needs such large memory sizesfor doing its job, and can be used to perform the scanning too. Thetable above indicates that a 128 MByte system scans up to A3 originals.RIP systems with 64 MByte memory, can scan A4 sized documents of anyrectangular image that has the same surface as an A4 page. A 32 MByteRIP system can scan originals with a surface equivalent to an A5document.

Yet another feature that a RIP system with moderate memory size canoffer, is real time preview and cropping. The data on an A3 sizeddocument can be scanned in one single pass and stored with a lowerresolution (e.g. 200 or 100 dpi). The processor means in the RIP systemtakes care of the subsampling. That resolution is enough to deliver apreview to the operator at an interactive workstation. The reducedresolution also reduces the transmission time between the RIP system andthe workstation, and requires no full 128 MB (e.g. 32 MB suffices for a200 dpi preview). The operator can then mark a rectangle to be scannedat full resolution. As long as the surface of that rectangle is lessthan the surface of an A4 page, all these operations can be done with a64 MByte RIP system.

If the RIP system has only 32 MByte and an A4 sized document must bescanned, the processor means in the RIP system can downscale--in realtime during scanning at 40 MByte/sec--to a lower resolution e.g. 300dpi, to fit the whole image in memory.

The whole process of preview and cropping can thus be done in twopasses, where other systems might need six passes.

Although the present invention has been described with reference topreferred embodiments, those skilled in the art will recognise thatchanges may be made in form and detail without departing from the spiritand scope of the invention.

We claim:
 1. A method in producing a color image by printing a pluralityof superposed monochrome images on a sheet (37), using an appropriatecolor for each of the monochrome images, whereineach of the monochromeimages is composed of microdots (36) each having an address (x,y), eachof the microdots (36) is represented by a pixel (32) carryinginformation about the address (x,y) and an image signal (I_(x),y), themicrodots (36) are partitioned by a screen (40) into identical screencells (33) composed of M microdots (R_(i)), M being a predeterminedinteger, each of the microdots (R_(i)) has a corresponding pixel tonecurve (L_(i), 34); the method comprising, for each of the pixels(32):(i) transforming the image signal (I_(x),y) by the correspondingpixel tone curve (L_(i), 34) into a 4-bit bitmap signal (B), and (ii)using the bitmap signal (B) in forming a P-bit printer signal, where Pis a predetermined integer, for printing the microdot (36) at theaddress (x,y).
 2. The method of claim 1 in producing a color image byelectrophotographic printing.
 3. The method of claim 1 in producing acolor image by inkjet printing.
 4. The method of claim 1, wherein P hasa value of
 6. 5. The method of claim 1, wherein P has a value of
 8. 6.The method of claim 1, wherein P has a value of
 4. 7. The method ofclaim 1, 2, 3, 4, 5 or 6, carried out whilst superposed monochromeimages for a preceding color image are being printed.
 8. A method inproducing a color image by printing a plurality of superposed monochromeimages on a sheet (37), using an appropriate color for each of themonochrome images, whereineach of the monochrome images is composed ofmicrodots (36) each having an address (x,y), each of the microdots (36)is represented by a pixel (32) carrying information about the address(x,y) and an image signal (I_(x),y), the microdots (36) are partitionedby a screen (40) into identical screen cells (33) composed of Mmicrodots (R_(i)), M being a predetermined integer, each of themicrodots (R_(i)) has a corresponding pixel tone curve (L_(i), 34); themethod comprising, for each of the pixels (32):(i) transforming theimage signal (I_(x),y) by the corresponding pixel tone curve (L_(i), 34)into a 2-bit bitmap signal (B), and (ii) using the bitmap signal (B) informing a P-bit printer signal, where P is a predetermined integer, forprinting the microdot (36) at the address (x,y).
 9. The method of claim8 in producing a color image by electrophotographic printing.
 10. Themethod of claim 8 in producing a color image by inkjet printing.
 11. Themethod of claim 8, wherein P has a value of
 6. 12. The method of claim8, wherein P has a value of
 8. 13. The method of claim 8, wherein P hasa value of
 4. 14. The method of claim 8, 9, 10, 11, 12 or 13, carriedout whilst superposed monochrome images for a preceding color image arebeing printed.
 15. A method for printing a plurality of color imagescomprising printing, for each of the color images, a plurality ofsuperposed monochrome images on a sheet (37), using an appropriate colorfor each of the monochrome images, whereineach of the monochrome imagesis composed of microdots (36) each having an address (x,y), each of themicrodots (36) is represented by a pixel (32) carrying information aboutthe address (x,y) and an image signal (I_(x),y), the microdots (36) arepartitioned by a screen (40) into identical screen cells (33) composedof M microdots (R_(i)), M being a predetermined integer, each of themicrodots (R_(i)) has a corresponding pixel tone curve (L_(i), 34); themethod comprising, for each of the pixels (32):(i) transforming theimage signal (I_(x),y) by the corresponding pixel tone curve (L_(i), 34)into a 4-bit bitmap signal (B), and (ii) using the bitmap signal (B) informing a P-bit printer signal, where P is a predetermined integer, forprinting the microdot (36) at the address (x,y); wherein (i) and (ii)are carried out whilst superposed monochrome images for a preceding oneof the color images are being printed.